Best Way To Reduce A Tube Diameter Gradually?

As some of you may have noticed, the "Scale" Seawolf Kits can never actually be Scale, do to the fact that on the Actual Missile, the Tube begins to narrow where the forward Fins stop. Also the Nose Cone that comes with Kits is typically an Ogive and not true to Scale either, but that part is easy, as a Lathe can be used to turn a Cone.
My question to those who care to entertain it is, what would be the best way to get that reduction in Diameter for the top end of the Body Tube?
I immagine that cutting it and removing some then squeezing it together and papering over the Seams would work???
I want to do a 2.6" to fly, then later get the Madcow 4" and build it, but wait to get my L1 to fly it.

The second Pic' shows this Diameter Step Down quite well.

Roll your own

I would think this is doable. I would build the section with the taper with an inner tube. Attach foam on the outside and cut down to desired shape/size. When done put a layer of glass on it.


Mark Koelsch
Sent from my iPhone using Rocketry Forum

Yes, you can reduce the diameter of a larger tube, so there's little reason that you couldn't also induce a taper into that tube. Folks have been reducing tube diameters for a long time by calculating the circumference of the desired tube diameter and the circumference of the tube they're wishing to reduce the diameter of, and then cutting a "slit" of material lengthwise out of the tube. Usually the piece removed is used to reinforce the joint by gluing it underneath the slit, with half overlapping each side of the slit. No reason that slit can't be cut into a taper, and that tapered piece that is removed used to reinforce the joint afterwards (from the inside).

The key is to calculate the circumference of the desired diameter at the upper end of the tube, and the diameter of the tube as it presently exists, and subtract the two, to give the upper end width of the slit to be cut out of the tube. Same goes for the lower end of the tube, if it's not the same diameter as the lower (larger) diameter you're starting out from. Then the tube has to be marked with a vertical line (reference line, which would be used as the cutting line if you were just reducing the diameter of the tube but keeping it cylindrical... since the tube is being tapered, you'd want to use it as a "centerline" or reference line, and then mark half the difference in the circumference on either side of this reference line at the upper end, and any difference in diameter halved on either side of the reference line at the bottom, and then connect these two lines down the length of the tube to establish the tapered cutting line. Then this strip should be cut out along the two tapered cutting lines on either side of the reference line.

Glue the tapered strip along the reference line to one side of the cut, behind the cut on the inside of the tube, and then bring the tube together and glue the other side to the other half of the cut off strip, pulling the joint together snugly and taping it shut or clamping it or whatever is necessary to ensure that everything stays tightly together. A little pre-curling of the tube is probably also a good idea...

Later and good luck! OL JR

Well, here goes!!! I'm starting my first attempt tonight. I'll be using the Method outlined by Luke. Wish me luck!

Alright! This can definitely work!
Anyhow, hear is what I have learned so far.

I started by marking the Tube, and masking it off.



I got a fresh Blade for my Hobby Knife and began scoring the Tube using the Masking Tape for guidance. I realized immidiately that I'de have to only score the Lines first, as trying to cut them all at once would be near impossible due to the lack of rigidity with each succesive cut.



The scoring alone was weakening the Tube to the point that accurate passes were becoming difficult, so I stuck a Nose Cone in and taped it in place to secure the Work.



Finally I had cut away the "Sections".



I removed the Tape, and sanded away the rough Edges where I had made my Cuts. Then I used some more Masking Tape to hold the "Petals" together to get an Idea of where I was going. This creased the Tube slightly at the Junctures of the Cuts where the Original BT Diameter resumed. This was due in part to the stiffness imparted to the Paper by the Glassine Layer, so I began to rough that up a bit with some 80 Grit to make it more maleable.





That's about as far as I'm going to go tonight.
As you can see from this final Pic', it is indeed a viable Method.




If this works as well as I think it is going to, it will open up a whole bunch of Scale Designs that previously could only be dreamed about due to the lack of properly shaped Tubes.
Papering over the Seams and Bondo Putty will make this lots of fun!!! I'm so happy!!!
A Proper Scale Seawolf can now be had. I may just skip the part where I buy the TLP Kit, and just calculate some Dimensional Data myself.

Glad to help... good going!!!

KUTGW!! OL JR

What a perfect use for Super Thin CA. Rather than make a mess trying to glue "Tabs" into place, and having to clamp them and such, I simply put on some Nitrile Gloves and held the seams together while adding Super Thin CA. This locked the Tube into position. I then gave it a good scuffing with 80 Grit. This helped to fill in any remaining Gaps in my Seams, so to reinforce them, I applied the CA a second time, so that the Sanding Dust that had filled in the Seams would harden like a Cement, furthering the strength of the Bond.
It is quite Rigid now, but there is no point in removing the Masking Tape that I put inside initially, so it will stay to add to the overall integrity of the Structure.

I went ahead and made Paper reinforcements for the Seams out of 110# Cardstock, and applied them with TB II.
I then went ahead and used some Vinyl Tape to mask off a nice straight Line for where I would make my cut. This beng necessary to achieve the correct Diameter that I had chosen based upon the available Tubes I had to use as the Internal Tube. Calculating the desired Diameter of the Tube beforehand using Math was not going to happen because I don't use Math.
I made my cut and inserted my Inner Tube for dry fitting.





This is obviously not the Cone I will use, just what I had on hand to test fit.

I Papered the Areas inbetween the Paper Reinforcements, and let that dry under heat, then filled the entire area with Bondo to fill the Seam lines.
It has to sit outside because of the Fumes, and it is still cold out, but it should be set up in a couple of hours once the Sun gets on it.
I'll sand that down nice before I paper the rest of the Tube. After that all I'll need to do is Bondo and sand again. Then I can use this Part to begin assembling my Seawolf. There will not be a Build Thread, but hopefully we all learned something from my Experience making this Part.
I'll post one more time in this Thread showing how it looks after it is sanded.

There's no mention of it, but it seems like you would get the best results by making multiple slits, rather than just one (3 or more). Think of this as using the bulkhead-and-ribs/stringers construction method sometimes used in balsa models. Of course, there's a practical limit, and sometimes you can only make the one cut.

sooner.boomer said:

There's no mention of it, but it seems like you would get the best results by making multiple slits, rather than just one (3 or more). Think of this as using the bulkhead-and-ribs/stringers construction method sometimes used in balsa models. Of course, there's a practical limit, and sometimes you can only make the one cut.

Click to expand...

I made 4 Slits. Can't you tell in the Pics'???

Anyway,...
Just like when you use Bondo Putty for Spirals, this application required a second application of the stuff, and ofcourse one to fill the miniscule Seam on my Cardstock Wrap. And I do mean Miniscule, as I'm getting pretty good at Papering things.

TopRamen said:

I made 4 Slits. Can't you tell in the Pics'???

Click to expand...

Yes, now that you mention it, I now see that you did. It wasn't specifically called out in Luke's instructions (or did I miss that too?). Good thinking on your part!

sooner.boomer said:

Yes, now that you mention it, I now see that you did. It wasn't specifically called out in Luke's instructions (or did I miss that too?). Good thinking on your part!

Click to expand...

Right, yeah, I don't think that Luke mentioned making Multiple Slits. I guess I just assumed that it was implied, or inferred or whatever the correct term, but looking back at his post he does'nt mention it.
That said, for anyone that is thinking about doing this, if I had to do it again, and when I do it next time, I will use atleast 5 Slits, as only using 4 is what caused me to have to deal with the amount of Creasing that I did. 5 would work better, and I immagine that 6 would be even better than 5.

TopRamen said:

Right, yeah, I don't think that Luke mentioned making Multiple Slits. I guess I just assumed that it was implied, or inferred or whatever the correct term, but looking back at his post he does'nt mention it.
That said, for anyone that is thinking about doing this, if I had to do it again, and when I do it next time, I will use atleast 5 Slits, as only using 4 is what caused me to have to deal with the amount of Creasing that I did. 5 would work better, and I immagine that 6 would be even better than 5.

Click to expand...

You're right... I didn't specifically mention it... I wanted to keep the math as simple as possible... (circumference of the tube as-is minus circumference of the tube diameter you desire... divide that difference by the number of slits you plan to use). You could use any number of slits you want, but the more you make, the more you have to fix and the more careful you have to be... plus the more you weaken the structural integrity of the tube... Personally I'd think four is probably enough, depending on the size of the tube... it also really depends on how much diameter difference you intend to make and how rapidly the tube "necks down" (how steep the taper of the tube). The greater the diameter difference and the more rapidly the tube tapers, the more slits one would need to get a good smooth result.

Basically it's like papermodeling... folks that get accomplished at the technique can actually make pretty nice domes or hemispheres out of flat paper rolled into a "crown" shape with lots of slits in it tapering to a point...

Later and good luck! OL JR

So, got it all sanded to my liking, then sealed the whole thing up with Finishing Epoxy. If I decide to use this as a Working Model, I'll simply sand through it and the Primer which I'm going to put on it and cut crosshatches if I need to attach Fins or whatnot.
It's not like my Seawolf Model is going to break Mach 1 on a 2 or 3 24mm Motor Cluster. I may even attach the Front Fins with Angle Brackets and Screws like I have seen on some Variants of the Missile. You can see Daylight between the Airframe and Fins on one of them.:eyepop:
This one:


That's one slick lookin' Seawolf!

Actually, I believe this is an Inert "Training Round". I just barely noticed that the Rear Fins have no Steering Pintles/Actuators.

What's with all the blood, man?
Are you really that much of a fumble fingers with sharp knives?:wink:

Pretty neat!

If I were to try this, I would tack the pieces together with CA and then wrap with some thin, light weight fiberglass cloth. Probably 2 or 3 wraps of .75oz. and use finishing epoxy Then maybe a tiny bit of filler to help hide the seams.

Also if you use peel-ply you would have far less sanding.


Jerome

JeromeK99 said:

Pretty neat!

If I were to try this, I would tack the pieces together with CA and then wrap with some thin, light weight fiberglass cloth. Probably 2 or 3 wraps of .75oz. and use finishing epoxy Then maybe a tiny bit of filler to help hide the seams.

Also if you use peel-ply you would have far less sanding.


Jerome

Click to expand...

I aware of the Fiberglass Option, and yes I too feel that that would be the prefered Method, but for me that is not an option just yet. My Workbench is right in my Tiny Livingroom, and too small to accomodate anything more than I already have going on right now. I do have a Work Shop outdoors, but even then, I'de have to get the Materials and set up a spot amongst the Bicycle and Lawnmower/Chainsaw Parts.
I do intend to get into FG sometime next year.

Is there any reason you can’t use a paper shroud type reducer?

boomtube-mk2 said:

Is there any reason you can’t use a paper shroud type reducer?

Click to expand...

No, that can be done easily and is the common Practice, but I wanted to see if this would work too.

Thats gonna be a really cool looking bird

This Technique will make for some easy Tail Cones too.